This research program is designed to (1) study the development of the sensory epithelial membranes of the inner ear and the cytodifferentiation of hair cells at the three phases of embryonic development, and (2) to elucidate the factor(s) influencing the histogenesis and cytodifferentiation of the hair cells, using the embryonic chick as a model. This is a light and electron microscopy study: (1) Phase 1. This is the proliferative phase. 4-day old embryonic chick otocysts, 4-day old cultures, and 8-day in vivo specimens are the representative specimens of this phase being investigated. Phase 2. This is the mid-point in development. 8-day old cultures and 12-day in vivo specimens are under investigation. Phase 3. Completely formed and differentiated sensory epithelial membranes of the inner ear have formed. 12-day old cultures and 16-day in vivo specimens are representative of this phase. (2) The following experiments are in progress or planned: a. A study of the 4-day old embryonic chick otocyst treated with Ca- and Mg- free salt solution and trypsin revealed that a model had been created in which the basement lamina had been destroyed and the intercellular connections altered. b. A study of the 4-day old cultures of these treated otocysts revealed that the basement lamina had been restored and that the epithelium had reorganized and continued to proliferate. Eight and 12-day cultures will also be studied. c. Experiments are designed to separate the tissue components from the otocyst (epithelial sac) and to recombine the mesenchyme with and without the acoustic ganglion.